Products made from cellulosic fibers are desirable because they are biodegradable, are made from a renewable resource and can be recycled. The main drawback is that the typical cellulosic product has a relatively high density or low bulk. Bulk is the reciprocal of density and is the volume occupied by a specific weight of material and is designated in cm.sup.3 /g. The amount of cellulosic material required to provide the requisite strength creates a heavy product. It has poor heat insulating qualities.
A 1990 brochure from Weyerhaeuser Company describes a chemically crosslinked cellulosic fiber known as High Bulk Additive or HBA and indicates the HBA fibers may be incorporated into paperboard at levels of 5% and 15%. The brochure also indicates that HBA can be used in the center ply of a three-ply paperboard. The board was compared with a conventional three-ply board. The basis weight was reduced 25%; the Taber Stiffness remained constant; but the breaking load was reduced from 25 kN/m to 16 kN/m in the machine direction and from 9 kN/m to 6 kN/m in the cross direction.
Knudsen et al U.S. Pat. No. 4,913,773 describe a product that has increased stiffness without an increase in basis weight. It is a three-ply paperboard mat. The middle ply is of anfractuous fibers. The two exterior plies are of conventional fibers. This structure, containing a middle ply of all anfractuous fibers, is compared with single ply mats of conventional and anfractuous fibers and double and triple ply constructions of different conventional fibers. Although in the comparison the middle ply is all anfractuous fibers, Knudsen et al also propose constructions in which the middle ply combines conventional and anfractuous fibers. In this latter construction Knudsen et al requires at least 10% by weight of anfractuous fibers in the center ply in order to obtain the necessary stiffness.
Knudsen et al obtain the anfractuous fibers by mechanical treatment, by chemical treatment with ammonia or caustic or by a combination of mechanical and chemical treatment. The treatment proposed by Knudsen et al does not provide intra-fiber crosslinking. Knudsen et al may use bonding agents with certain multi-ply constructions, using 1 weight percent starch to obtain adequate bonding of the plies.
Kokko European Patent No. 0 440 472 discusses high-bulk fibers. The fibers are made by chemically crosslinking wood pulp using polycarboxylic acids. Kokko is directed to an individualized crosslinked fiber, and single ply absorbent and high-bulk paper products made from this fiber.
Kokko used a blend of 75% untreated fibers and 25% treated fibers. The maximum dry bulk achieved by Kokko was 5.2 cm.sup.3 /g using 25% citric acid treated fibers and 5.5 cm.sup.3 /g using 25% citric acid/monosodium phosphate treated fibers.
Kokko also states that polycarboxylic acid crosslinked fibers should be more receptive to cationic additives important to papermaking and that the strength of sheets made from the crosslinked fibers should be recoverable without compromising the bulk enhancement by incorporation of a cationic wet strength resin. There is no indication that Kokko actually tried cationic strength additives, or any strength additives, with the crosslinked fibers. Consequently, Kokko did not describe the amount of cationic additive that might be used or the result of using the additive. Treating anionic fibers, such as Kokko describes, with a cationic additive substantially completely coats the entire surface of the fiber with additive. This is noted by Kokko in the experiment with methylene blue dye. The cationic additive is attracted to the entire surface of the anionic fiber. More additive is used than is needed to provide binder at the fiber-to-fiber contact points because the entire fiber is coated.
Young et al U.S. Pat. No. 5,217,445 discloses an acquisition/distribution zone of a diaper. It comprises 50 to 100% by weight of chemically stiffened cellulosic fibers and 0 to 50% by weight of a binding means. The binding means may be other nonstiffened cellulosic material, synthetic fibers, chemical additives and thermoplastic fibers. The material has a dry density less than about 0.30 g/cm.sup.3, a bulk of 3.33 cm.sup.3 /g.